At a press conference in March the Brexit Secretary David Davis and the EU’s chief negotiator Michel Barnier unveiled a slideshow of the Brexit Treaty, with the sections agreed by both sides highlighted green.

It amounted to roughly 75-80% of the 129-page document that will seal the terms of the UK’s departure from the EU and which is officially called the “Agreement on the Withdrawal of the United Kingdom”.

The UK says the deadline for settling the outstanding issues is October, when the agreement will be submitted to the European Parliament. Mr Barnier wants progress on these issues in time for an EU summit on 28 June.

Here is what’s left to discuss:

Northern Ireland

Both sides have committed to avoid infrastructure on the border between Northern Ireland and Ireland.

Brussels has proposed a protocol – nicknamed “the backstop” – which would see Northern Ireland stick to those rules of the customs union and single market that are required for cross-border co-operation to continue.

It’s described as an insurance policy in case no other solutions are found.

Britain agrees to the need for a backstop but says this version risks barriers being created between Northern Ireland and the rest of the UK, and so wants an alternative.

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Media captionTechnology can streamline EU borders but they are not frictionless.

Solving disputes

Known in the trade as “governance” this is an item that sounds boring but which Michel Barnier says could bring the whole deal down: how to solve disputes between the UK and the EU arising as a result of the treaty?

The EU has proposed a Joint Committee made up of representatives appointed by London and Brussels. If they can’t solve a problem, it would be referred to the European Court of Justice.

The UK government likes the idea of “J-Com” but not of judges in Luxembourg having the final say.

A complicated compromise to oversee the governance of the citizens’ rights part of the deal hasD been agreed, though.

Then there is a swathe of subjects known as Other Separation Issues (OSI’s).

Champagne and pasties

The EU is proud of its rules that protect regional products so that champagne can only come from Champagne, Manchego cheese can only come from La Mancha, Cornish pasties from Cornwall.

Brussels wants this system written into British domestic law. The Brits agree with the concept but aren’t sure about the method and want to ensure it applies to British products in the EU too.

Some in the food and drink industry suspect some ministers view this as a measure to protect European industries that would limit the UK’s room for manoeuvre in future trade talks with other countries.

Crime and courts

There are still disagreements about how both sides will work together on on-going police and judicial matters.

For example, the EU says that the European Court of Justice should be able to pass judgements that have an effect in the UK after the end of the transition period in cases where events occurred before the end of the transition period.

On security, the UK thinks that co-operation on extradition, the British relationship with the EU crime-fighting agency Europol and the sharing of criminal records should be the subject of a separate security treaty which the two sides should begin negotiating straight away.

Data

British companies hold all sorts of personal data belonging to EU citizens, and the European Commission thinks that European data protection law should continue to apply to it after Brexit.

The UK would like a comprehensive deal on data sharing with the EU as part of the discussions about the future relationship, and is wary of agreeing divorce-related measures that could tie its hands.

Government contracts

There’s a section about how to handle government tenders for goods and services which will be underway during the Brexit process – so called “public procurement”. Think new British passports being made by a Franco-Dutch firm.

Most of it has been agreed, but one jargon-filled paragraph stands out as unresolved.

It concerns the rule which says new contracts should be open to companies across the EU. Public procurement lawyers suspect the UK is waiting for a guarantee that British firms will be able to bid for European government business if British contracts remain open to their continental competitors.

Sharing information about tax

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The EU wants the UK to share customs data for three years after the end of the transition period, and information about certain taxes for five years after the end of the transition period.

And there are some other unresolved technical issues scattered throughout the document that test the knowledge of even the most seasoned Brexit geek.

But small details could have a big effect because the whole lot has to be agreed for the deal to be signed off.